According to one aspect of the invention, there is provided a method of absorbing a selected gas component from a gas stream which comprises: bringing the gas stream into contact with a liquid including a solvent or a reagent for the selected gas component in a turbulent contactor, the contactor including a gas inlet, a liquid inlet, an outlet leading to a venturi passage and a tube extending from the outlet back upstream, the tube being perforated and/or being spaced from the periphery of the outlet; subjecting the gas stream and the liquid to turbulent mixing conditions in the contactor thereby causing the gas component to be absorbed by the solvent or reagent.
The invention also extends to the apparatus for carrying out this method.
The turbulent mixing is very intense and results in extremely efficient gas liquid contact. The mixing regime is preferably turbulent sheer layer mixing. The liquid entrained in the gas may be in the form of droplets for gas continuous fluid phase distribution The efficient mixing means that absorption can take place very rapidly and in a relatively small amount of solvent compared to that required in conventional absorption columns. This in turn means that the liquid duty in the equipment is dramatically reduced resulting in a consequential reduction in the size of any downs regeneration section. At the same time, the mixing system used is simple and inexpensive compared to prior art systems, leading to reduced costs. Finally, an efficiency of approaching 100% for the removal of the selected gas component (e.g. acid gas from natural gas or combustion gas) can be achieved, for certain applications.
In addition, conventional absorption methods involve the evolution of heat which must then be removed from the system While the method of the invention is capable of operation with a relatively low pressure drop across the mixing means, when a greater pressure drop is employed, a cooling effect is achieved and this may render the need for additional cooling unnecessary.
The absorption may be achieved by simply dissolving the gas component or by way of a chemical reaction with the solvent.
Preferably, the method is carried out as a continuous process with the gas stream and liquid flowing currently. The co-current flow eliminates the problems associated with foaming, since separation can easily be effected downstream of the mixer.
Preferably, the method further includes the step of separating a gas phase and a liquid phase after the turbulent mixing. Preferably, the liquid phase is subsequently treated to remove the absorbed gas component
The turbulent mixing may be achieved by any convenient means, preferably in a turbulent contactor comprising a vessel having a gas inlet, a liquid inlet and an outlet leading to a venturi passage, and a tube extending from the outlet back into the vessel, the tube being perforated and/or being spaced from the periphery of the outlet. In one regime, the gas stream is supplied to the tube optionally directly and the liquid is supplied to the vessel, whereby the gas stream draws the liquid into the venturi and the two phases are mixed. In another regime, the gas stream is supplied to the vessel and the liquid is supplied to the tube optionally directly, whereby the gas stream is drawn into the venturi by low pressure generated by the flow through the venturi, and the two phases are mixed. Alternatively, the liquid and the gas stream are both supplied to the vessel, the liquid being supplied to a level above the level of the outlet, whereby the gas stream is forced out through the outlet via the tube, thereby drawing the liquid into the venturi so that the two phases are mixed.
The tube being spaced from the periphery of the outlet means that the phase passing via the tube draws the phase in the vessel at the outlet into the outlet via the space between the tube and the outlet Such a vessel is supplied by Framo Engineering A/S and is described in EP-B-379319. In the case where the tube is not spaced from the outlet, the tube is perforated and is arranged such that all the fluid which passes through the outlet does so by way of the tube.
It will be appreciated that the invention is applicable to any absorption application where the reaction kinetics are rapid, for example, the absorption of acid gas. The invention is also applicable to chemical reactions with fast reaction kinetics, where good mixing of the reactants is a requirement.
According to a more specific aspect of the invention, there is provided a method for removing a single selected component from a mixture of gases. Alternatively, the method extends to removing a plurality of gas components from a gas stream, either using a common solvent or reagent, or by respective solvents or reagents. According to a further aspect of the invention, the gas stream is a single gas which is absorbed.
Preferably, the gas stream and the liquid are formed into a homogeneous mixture in the contactor, the homogeneous mixture being cooled prior to separation into a gas phase and a liquid phase. Optionally, this phase separation occurs in a hydrocyclone.
Preferably, the solvent or reagent in the liquid phase is subjected to a regeneration treatment to remove the absorbed selected gas component. Preferably the regenerated solvent-containing liquid phase is recycled to the contactor.
Preferably, the regeneration is carried out by heating and/or by flashing off the absorbed gas component in a flash tank Preferably, the post mixing cooling and the regenerative heating are achieved at least in part by mutual heat exchange. Preferably, in instances where the gas stream is at a low pressure, the liquid is pumped to the vessel and thereby draws the gas stream with it through the vessel. Preferably, when the gas stream is at high pressures, it is conveyed to the vessel at a high pressure and thereby draws the liquid with it through the vessel.
The invention also extends to apparatus for carrying out such a method, comprising: a turbulent contactor having a liquid inlet, a gas inlet and a fluid outlet; a cooler for the fluid stream from the fluid outlet; a hydrocyclone arranged to separate the cooled fluid stream into a gas phase and a liquid stream; a regenerator arranged to treat the separated liquid steam; and a recycle line arranged to convey the regenerated liquid stream to the contactor.
The apparatus may include a pump arranged to supply liquid to the liquid inlet of the contactor. Preferably, the regenerator is a heater and/or a flash tank.
The invention may be considered to extend to the use of a turbulent contactor including a gas inlet, a liquid inlet, an outlet leading to a venturi passage and a tube extending from the outlet back upstream, the tube being perforated and/or being spaced from the periphery of the outlet for absorbing a selected gas component from a gas stream by bringing the gas stream into contact with a liquid including a solvent or a reagent for the selected gas component, thereby causing the gas component to be absorbed by the solvent or reagent.
Preferably, the tube is located in a vessel the vessel including the gas inlet, the liquid inlet and the outlet
Suitable solvents for use in the method of the present invention include amines such as MDEA, MEA and DEA and mixtures of solvents. Also suitable as a solvent is seawater, although in this case it is not necessary to regenerate the solvent after it has passed through the contactor.
The separation/absorption/reaction systems described are single operations, however it will be appreciated that multi separation/absorption/reactions may be performed. These may be carried out simultaneously or sequentially and may also be carried out in series or in parallel.
It will be appreciated that the methods and the systems described above may be used to selectively remove one or more gas components from a gas stream. Selective absorption may be generated by adjustment of the residence time through the system. Since the rates of reaction for absorption of a variety of gases by a particular solvent will vary, it is possible to selectively absorb one gas in preference to another. An example of this is the selective absorption of H.sub.2 S in an amine, which is virtually instantaneous, in preference to CO.sub.2 which is absorbed slower.
The improved efficiency possible for the removal of, for example, acid gases makes the present invention particularly valuable as awareness is increased of the potential damage to the environment that can be caused by acid gases in effluents such as combustion gas.
Furthermore, the small size of the apparatus compared to conventional absorption columns render the invention especially applicable to use in marine applications, such as on board shuttle tankers.